JP2014226689A - Method for evaluating cracking in thin plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform estimation of cracking in a thin plate considering the temperature at which breaking strain becomes maximum due to an influence of strain-induced martensitic transformation of austenite (γ) during being worked, of steel material containing the austenite (γ).SOLUTION: A method for evaluating cracking in a thin plate comprises: a step of collecting data of breaking strains εupon normal temperature Twith respect to a plurality of strain ratios ρ for predetermined material and making a function which calculates the breaking strain εupon normal temperature at an arbitrary strain ratio ρ; a step of respectively collecting a plurality of data of temperature T and breaking strain εwith respect to a plurality of strain ratios ρ; a step of making curves which satisfy the plurality of data of temperature T and breaking strain εfor each strain ratio ρ; a step of fitting one-variable symmetrical function to any one of the plurality of curves; and a step of making warm FLD based on the one-variable symmetrical function.

Description

  The present invention relates to a forming simulation method, a forming simulation apparatus, a program, a recording medium, and a forming method based on a simulation result, and more particularly to a breakage determination method and a breakage determination apparatus in a press forming simulation of a thin plate.

  In automobile parts and household electrical products, parts obtained by press forming a thin metal plate such as a thin steel plate or an aluminum thin plate into a predetermined shape have been widely used. When processing parts used for automobile parts and household electrical products, for example, a thin metal plate such as a thin steel plate or an aluminum thin plate is press-formed into a predetermined shape using a pair of upper and lower concave and convex molds.

  FIG. 8 is a schematic diagram of press molding. A workpiece 5 (sometimes referred to as “blank”) 5 is placed on a die 1, and the workpiece 5 is pressed by a blank holder 2, and punch 3 Is processed by moving in the punching direction 6 during processing, while the draw bead 4 holds the workpiece so that it does not flow during processing.

  FIG. 9 is a schematic diagram of a tensile test. For example, a tensile test piece 12 such as JIS No. 5 or JIS No. 13B is grasped at both ends by the grip portion 11 of the tester and pulled in the direction of the tensile direction 14. At that time, the load applied to the tensile test piece 12 is measured by a load cell (not shown), and the displacement of the test piece that changes every moment is measured at a part of the evaluation distance 13 by a contact-type displacement meter or an image measurement device.

  Now, in order to reduce the weight of automobile parts, reducing the plate thickness by using a material with higher strength has been actively carried out, and the material fracture phenomenon, which is a molding defect that occurs at that time, is predicted before processing. It is very important, and today, it is routinely performed to predict cracks using simulation calculation by a finite element method or the like.

  For example, Patent Document 1 discloses that a conventional elastic-plastic material forming simulation method using a finite element method divides a part into a plurality of regions in advance, and only a divided region is a large-scale simultaneous equation solution springback. It is necessary to repeat the calculation, and the work of identifying the part that causes the springback is complicated, or the result varies depending on the division method (size, number of divisions), and the part that causes the springback occurs. Calculating an element equivalent point force vector from a stress tensor for each of one or more finite elements in the target shape of an elastoplastic material, with the goal of solving the problem of being difficult to specify sufficiently; The calculated element equivalent nodal force vector for each finite element or elements is integrated over the good or specific area of the elastoplastic material and It discloses a forming simulation method of elasto-plastic material using a finite element method comprising the step of calculating a total equivalent contact vector of.

  In recent years, simulations such as those disclosed in Patent Document 1 have been performed, while limit strains such as sheet thickness limit lines and forming limit diagrams (hereinafter referred to as FLD (Forming Limit Diagram)) have been experimentally or theoretically calculated. Thus, by comparing the critical strain state with the strain state obtained by the simulation disclosed in Patent Document 1, the crack occurrence location and the occurrence time point are predicted quantitatively to some extent. .

  Here, an outline of the FLD measuring apparatus and the method of creating the FLD will be briefly described.

  FIG. 10 is a schematic diagram of an FLD measuring instrument.

  FIG. 10 shows an FLD measuring instrument, 21 is a die, 22 is a blank holder, 23 is a punch, 24 is a draw bead, 25 is a workpiece, 26 is a punch direction during processing, 27 is a constant temperature bath, 28 is a heater, 29 is silicone oil, and 30 is a blank holder pressing spring.

  In order to deepen the understanding, FIG. 10 will be described in comparison with the schematic diagram of press forming in FIG. The die 21 (FIG. 10) corresponds to the die 1 (FIG. 8), the workpiece 25 (FIG. 10) corresponds to the workpiece 5 (FIG. 8), and the punch 23 (FIG. 10) corresponds to the punch 3 (FIG. 8). The blank holder 22 (FIG. 10) corresponds to the blank holder 2 (FIG. 8), and the draw bead 24 (FIG. 10) corresponds to the draw bead 4 (FIG. 8).

  Further, the schematic diagram of the FLD measuring apparatus in FIG. 10 is different from the schematic diagram of press molding in FIG. 8 in that the workpiece 25 is immersed in a constant temperature bath 27 having silicon oil 29 so that the temperature is kept constant. In addition, the temperature can be raised by the heater 28.

  An outline of creation of FLD will be described with reference to FIGS. 9, 10, 11, and 12. FIG.

  FIG. 9 is a schematic diagram of the tensile test, and FIG. 11 is a diagram illustrating the test piece 12 (FIG. 9).

As shown in FIG. 11 (a), 1) a pattern such as a square grid having a vertical length L and a horizontal length L is transferred to the test piece 12 in advance, and 2) as shown in FIG. When a crack or the like is found as a result of being pressed by the punch 23 by a test using an appropriate FLD measuring instrument, 3) measure the vertical length L ₁ and the horizontal length L ₂ of the cracked portion of the grid, and 4 ) When L ₁ > L ₂ , the maximum principal strain ε ₁ and the minimum principal strain ε ₂ are
ε ₁ = ln (L ₁ / L) (formula A1)
ε ₂ = ln (L ₂ / L) (Formula A2)
And calculate. Here, ln represents a natural logarithm.

Thus, FLD can be constructed by obtaining the values of ε ₁ and ε ₂ and plotting them as shown in FIG.

  However, it has been pointed out that the result of fracture determination differs depending on the mesh size of the FLD model.

On the other hand, in Patent Document 2, that is, as a result of measuring the transition of the breaking strain with respect to the gauge length for the iron standard JSC270D, the fracture limit strain increases when the gauge length is small, but the fracture occurs when the gauge length is large. Focusing on the fact that the limit strain becomes smaller, the maximum principal strain ε ₁ , the minimum principal strain ε ₂ , the material parameter α and the plastic strain ratio ρ at a specific gauge length.
ρ = ε ₂ / ε ₁ (Formula A3)
A method has been proposed for solving the problem that the fracture determination differs depending on the mesh size of the FLD model by calculating the fracture limit strain at an arbitrary gauge length from the function β (ρ).

  However, when forming a steel material containing austenite while suppressing the occurrence of constriction and breakage, it is necessary to consider the temperature at the time of forming.

For example, Patent Document 3, previously heated steel material heating furnace or the like to the above A _c3 point the austenite single phase region of 750 ° C. approximately to 1000 ° C. before press forming of the steel sheet, the steel of the state of the single-phase austenite A method of manufacturing a molded product with high strength and good dimensional accuracy is disclosed by press-molding, quenching and quenching the steel material using heat transfer from the steel material to the mold.

  Patent Document 4 discloses a method of drawing a steel material containing austenite while heating the die die and cooling the die punch, that is, a part of the steel material that becomes the flange portion after molding. While heating by heat transfer with the die to reduce its deformation resistance, other parts of the steel material are cooled by heat transfer with the punch to increase its deformation resistance and draw-molded, A method of drawing while preventing wrinkles and breakage is disclosed.

  Further, in Patent Document 5, the metal structure of a work material that is a steel material is 70% or more of bainitic ferrite and granular bainitic ferrite as a parent phase in terms of volume fraction, and retained austenite as a second structure. By controlling C in the retained austenite to 1.0 mass% or more, the total elongation value of the steel material, which is 7% at room temperature, becomes 20% at 250 ° C. A method for improving the moldability at that temperature is disclosed.

  Thus, although the importance of considering the temperature is shown in the molding methods disclosed in Patent Documents 3 to 5, the moldability is evaluated based on the molding simulation result using the finite element method. In the warm FLD model for performing, the temperature at the time of molding cannot be considered sufficiently.

  In Patent Document 6, the cracking characteristics of a material having high ductility are obtained by making the material easy to crack against the processing crack that occurs when a plastic material such as a metal is molded in a warm region or a hot region. For the purpose of enabling evaluation, a plastic test piece with an axisymmetric shape heated in advance from the warm range to the hot range is forged with a flat tool and the processing conditions under which cracks occur on the side of the test piece are compared. In the hot work crack strength test method for a plastic material for evaluating the work crack strength, a test piece having an annular protrusion formed on a cylindrical side surface is used as the axisymmetric plastic test piece. Although a hot working crack strength test method for plastic materials to be forged and forged has been disclosed, it has not yet been possible to accurately determine cracks in consideration of the temperature characteristics of steel materials containing austenite (γ).

WO2010 / 038539 JP2011-147949A JP 2005-177805 A JP 2007-1111765 A JP 2004-190050 A JP 09-248647 A

  The present invention has been made to solve the above-mentioned problems, and provides a method for creating a warm FLD and a warm strain stress curve in consideration of the temperature characteristics of a steel material containing austenite (γ). To provide a method for accurately evaluating warm formability (determination of occurrence of wrinkles and cracks) based on a forming simulation result using a finite element method in which a steel material containing austenite (γ) is processed. With the goal.

  As a result of diligent research and development, the inventors of the present invention have found that a steel material containing austenite (γ), such as TRIP steel or SUS steel, has the maximum breaking strain due to the effect of processing-induced martensitic transformation of γ during processing. Maximum fracture strain temperature, which is the maximum temperature, varies depending on the plastic strain ratio (hereinafter simply referred to as “strain ratio”) ρ, and the maximum strain value and the maximum strain strain temperature corresponding to the strain ratio ρ. It has been found that a high-precision crack determination of a thin plate can be performed by taking in the warm FLD.

In order to construct a warm FLD that satisfies such conditions, it is necessary to measure the strain ratio ρ and the breaking strain ε _f for various temperatures. It takes a lot of time and effort, and its feasibility is poor, but if the material is the same, it has one variable, convex upwards, only one maximum and inflection point, It was found that a breaking strain ε _f without an experimental value can be appropriately estimated using a function that can be expressed by a symmetric function with a straight line perpendicular to the variable axis as an axis through the value of the variable taking the value.

Furthermore, the inventors work hardening exponent n can be approximated as a linear function f ([rho) of Ibitsuhi [rho, breaking strain epsilon _f0 when the ambient temperature T _o is to be able to calculate the constructed expression by the distortion ratio [rho I found it.

  The inventors have completed the present invention based on these findings. The gist of the invention is as follows.

(1) A method for producing a warm FLD used for cracking evaluation of a steel material containing austenite (γ),
For each strain ratio of the steel material containing austenite (γ), the maximum value of the breaking strain (hereinafter referred to as “breaking strain maximum value”) and the temperature at which the breaking strain maximum value is obtained (hereinafter referred to as “breaking strain maximum temperature”). A method for evaluating cracking of a thin plate, comprising the step of creating a warm FLD having information on

(2) A method for producing the warm FLD,
About a steel material containing one predetermined austenite (γ),
A step of collecting a plurality of data of temperature T and breaking strain ε _f for each of a plurality of strain ratios ρ;
Creating a curve satisfying a plurality of data of the temperature T and the breaking strain ε _f for each strain ratio ρ;
The variable is one temperature, convex upward, has an inflection point and a single maximum value, the temperature that takes the maximum value changes from the strain ratio ρ, and passes through the temperature value that takes the maximum value. A step of fitting a symmetric function (hereinafter referred to as “one-variable symmetric function”) with respect to a straight line perpendicular to the temperature axis to any one of the plurality of curves (hereinafter referred to as “FLD function”). And)
Wherein the plurality of strain at break maximum temperature T _p and the curve distortion ratio curve [rho, creating a function T _{p ([rho)} of the Dan'ibitsu maximum temperature T _p fracture of the plurality of curves the Ibitsuhi [rho variable When,
(2) The method for evaluating cracking of a thin plate according to (1).

(3) A method for producing the warm FLD,
About the steel material containing one predetermined austenite (γ),
For a plurality of strain ratios ρ, collecting fracture strain ε _f0 data at room temperature T ₀ ;
Creating a function (hereinafter referred to as “room temperature rupture strain function”) for calculating a rupture strain ε _f0 at room temperature T ₀ when a strain ratio ρ is given from the data;
(2) The method for evaluating cracking of a thin plate according to (2).

(4) A method of creating the normal temperature breaking strain function,
A function f () having the work hardening index n as a strain ratio ρ variable from the data of the work hardening index n and the break strain ε _{f0 at} the normal temperature T ₀ from the plurality of strain ratios ρ and the breaking strain ε _f0 data at the room temperature T _0. creating ρ);
Using f (ρ) to create a normal temperature breaking strain function;
(3) The method for evaluating cracking of a thin plate according to (3).

(5) Using the warm FLD obtained by the method described in (1) to (4) and judging the warm formability based on the result obtained by the finite element method, the crack of the thin plate Evaluation method.

  According to the method of the present invention, it is possible to predict a thin plate crack with high accuracy in consideration of the fracture strain maximum value and the fracture strain maximum temperature of a steel material containing austenite (γ) such as TRIP steel and SUS steel. Has a remarkable effect. In addition, this effect becomes more remarkable in the case of a steel material containing 10% or more of austenite (γ) such as TRIP steel or SUS steel.

It is a figure showing the procedure of the thin-plate crack determination using a finite element method result. It is a figure showing a warm stress strain curve. It is a figure showing a warm FLD curve. It is a figure showing temperature T (degreeC) and a limit strain ((epsilon) _f ). It is an image figure of warm FLD curve creation. It is an image figure of the boundary conditions of press molding. It is a figure showing an Example (when a steel plate is substance A). It is a schematic diagram of press molding. It is a schematic diagram of a tensile test. It is a schematic diagram of the measurement of FLD. It is a figure explaining a test piece. It is a figure showing the example of FLD.

(First embodiment)
The first embodiment of the present invention is a method for constructing a warm FLD used for crack determination of a steel material containing austenite (γ) such as TRIP steel or SUS steel. Specifically, the following two steps are included. Consists of.

[Step group 1]
This is a step of creating a function f (ρ) using a work hardening index n as a strain ratio ρ variable for a predetermined material.

  The inventors have used the Stoeren-Rice formula

In the case of austenitic steel materials (TRIP steel, SUS steel), the work hardening index n depends on the plastic strain ratio ρ because the work-induced martensitic transformation is dependent on the plastic strain ratio ρ. The following steps were constructed by finding that the hardening index n is a function of the strain ratio ρ and the work hardening index n can be a function of the strain ratio ρ when the materials are the same.

(Step 1-1) This is a step of collecting data.
Experiments and the like are performed, and the fracture strain ε _f0 data at room temperature T ₀ is collected for a plurality of strain ratios ρ.

(Step 1-2) This is a step of calculating a work hardening index.
The strain ratio ρ obtained in the experimental building, etc. and the fracture strain ε _f0 data at room temperature T ₀

The work hardening index n obtained by substituting for is calculated.

(Step 1-3) This is a step of creating a function f (ρ).
A function f (ρ) having the work hardening index n as a strain ratio ρ variable is created from the data of the work hardening index n obtained by the calculation and the breaking strain ε _f0 at room temperature T ₀ .
For example, the function f (ρ) is

It is easy to obtain the function f (ρ) by using the linear function described as follows, but it may be possible to obtain better accuracy if the function f (ρ) is a general function of ρ.

(Step 1-4) This is a step of creating an equation for calculating the breaking strain ε _f0 .
Using the f (ρ), the breaking strain ε _f0 at the room temperature T ₀ at an arbitrary strain ratio ρ is calculated.

Create In this way, by giving the work hardening coefficient n in the formula of Stoeren-Rice as a function of [rho, breaking strain epsilon _f0 when highly accurate normal temperature _{T o} is obtained.

[Step group 2]
This is a step of creating an FLD function or the like for the predetermined material.

(Step 2-1) This is a step of collecting data.
An experiment or the like is performed, and a plurality of data of temperature T and breaking strain ε _f are collected for each of a plurality of strain ratios ρ.

(Step 2-2) is a step of creating a curve of temperature T and strain at break epsilon _f.
For each strain ratio ρ, a curve satisfying a plurality of data of temperature T and breaking strain ε _f is created. Usually, three types of curves are obtained: uniaxial tension (ρ = −0.5), plane strain tension (ρ = 0), and biaxial tension (ρ = 1.0).

The curve that satisfies the data refers to a curve that can be satisfied while allowing a predetermined error range for each data, and the error range is a range of ± 5 to 10%. A curved line includes a straight line, and the creation of the curved line includes a method using multiple regression, a spline curve, and a case where a curved line and a straight line are drawn by hand.
(Step 2-3) This is a step of fitting a function to the obtained curve.

  A curve having a maximum value of breaking strain and a maximum temperature of breaking strain, which is a feature of the present invention, will be described.

Since the breaking strain maximum value and the breaking strain maximum temperature vary depending on the strain ratio ρ, at least uniaxial tension (ρ = −0.5), plane strain tension (ρ = 0), biaxial tension (ρ = 1). 0.0), the data representing the relationship between the breaking strain ε _f and the temperature T are plotted on the graph of the judgment strain ε _{f with} the horizontal axis representing the temperature T (° C.) and the vertical axis representing the judgment strain ε _f . The work of confirming the maximum value of breaking strain and the maximum temperature of breaking strain is extremely important.

The relationship between the strain ratio ρ and the rupture strain maximum temperature T _p will be described with reference to FIG.

As shown in FIG. 4 (a), in steel materials containing austenite (γ) such as TRIP steel and SUS steel, the breaking strain at which the breaking strain takes the maximum value due to the effect of the processing-induced martensitic transformation of γ during processing. The maximum temperature becomes higher as ρ becomes larger. When ρ = 1.0, the maximum strain at break may be taken at 200 ° C. or higher, and when temperature T _i is up to 200 ° C. In some cases, the maximum strain value cannot be confirmed. In such a case, the maximum strain value and the maximum strain value for uniaxial tension (ρ = -0.5) plane strain tension (ρ = 0) 2 curve are set. The inventors have confirmed that a straight line satisfying these two points can be formed, and the maximum strain temperature at ρ = 1.0 can be estimated.

This is because, as shown in FIG. 4 (b), as the material A, material B, the straight line is different depending on the workpiece, breaking strain maximum temperature strain ratio [rho T _p is approximately linear relationship For example, in the case of material A, the inventors have found that the maximum strain at break of ρ = 1.0 can be estimated using a straight line derived from two points. However, it can also be estimated in detail using a curve other than a straight line.

  The temperature at which data is collected by experiment is preferably in increments of 50 ° C. or in increments of 25 ° C. in order to facilitate confirmation of the maximum strain value and the maximum strain value. If the maximum breaking strain value and the maximum breaking strain temperature cannot be confirmed even in steps, there is a high possibility that the temperature that is intermediate between the two temperatures at which almost the same breaking strain value is taken becomes the maximum breaking strain temperature. It is desirable to add experiments at temperature.

  Therefore, the curve obtained in Step 2-2 has basically the same shape except for the maximum strain value at which the maximum strain value and the maximum strain value are different. Since there is a curve where the maximum value cannot be seen, the inventors have found that it is only necessary to fit a curve that makes it easy to grasp the characteristics of the curve.

  The univariate symmetric function to be fitted is 1) one variable temperature, 2) convex upward, 3) an inflection point and a single maximum value, and 4) the temperature at which the maximum value is taken is the strain ratio ρ. 5) It has a property of being symmetric with respect to a straight line perpendicular to the temperature axis passing through the maximum temperature value.

The point of fitting is to match 1) the breaking strain maximum temperature T _p , 2) the breaking strain maximum value, 3) the position of the inflection point, and the breaking strain ε _f0 value at room temperature T ₀ .

For example, a one-variable symmetric function is described by a strain ratio ρ, a breaking strain ε _{f0 at} room temperature T ₀ , a temperature T _p (ρ) at which the breaking strain ε _f is maximum, and constants D, σ, and k. ,

In the case of T = T ₀ , since ε _f ≈ε _{f 0} ,
1) T _p (ρ) is the maximum temperature at which the breaking strain ε _f takes the breaking strain maximum value,
2) The temperature at which T _p (ρ) and the inflection point are taken is σ / k,
3) k is (2 × ln (2)) ^1/2 ≈1.2,
4) determining D from breaking strain epsilon _f0 Metropolitan at the maximum value epsilon _f0 (1 + D) and room temperature _{T o} of the breaking strain epsilon _f,
Thus, fitting is possible.
(Step 2-4) This is a step for obtaining T _p (ρ).

From the maximum temperature value T _p of the curve obtained in step 2-2 and the curve strain ratio ρ, the breaking strain maximum temperature T _p of the plurality of curves is a linear function with the strain ratio ρ as a variable,

Create When only two points are obtained, the function is obtained as a straight line connecting the two points, and when there are three points, it is obtained from a straight line obtained by the least square method.

(Creation of warm FLD)
First, the fracture strain ε _f at an arbitrary temperature T is obtained using the FLD function.
Second, since ρ is a strain ratio and ε _f is a fracture strain (= maximum principal strain ε ₁ ), the minimum principal strain ε ₂ is calculated using the obtained maximum principal strain ε _1.

Ask for.
Third, for each temperature T, the obtained maximum principal strain ε ₁ and minimum principal strain ε ₂ are plotted to create a warm FLD as shown in FIGS.

(Second Embodiment)
The second embodiment of the present invention is a method for performing crack determination on a thin plate using the warm FLD created according to the first embodiment.

A procedure for determining formability based on a press forming simulation result of a thin plate using the finite element method will be described with reference to FIG.
[Condition setting]
In the condition setting step, boundary condition setting and warm SS curve setting are performed.

  First, boundary condition setting will be described.

FIG. 6 shows an example of boundary conditions. The boundary conditions include, for example, basic boundary conditions such as a plate thickness of 1.4 mm, a temperature of 150 ° C., a blank holder force (BHF) of 100 tons, a friction coefficient of 0.12, a forming speed of 3 mm / msec, a punch, a draw bead, and a blank holder. And individual boundary conditions such as dies.
[Warm forming analysis]
In the warm forming analysis step, the stress / strain and the surface pressure are calculated by an elastic-plastic FEM from the warm stress-strain curve and boundary conditions.
[Analysis of formability]
In the warm forming analysis step, it is examined whether cracks and wrinkles are generated using a warm FLD model as shown in FIG. 3 created by the method of the present invention based on the calculated stress / strain.

The Example about preparation of warm FLD which is 1st Embodiment is demonstrated.
<Fitting of (Formula 5)>

Perform fitting.

Calculated by the data-point experiments of breaking strain epsilon _f0, the abscissa distortion ratio [rho, is a vertical axis work hardening coefficient n graph diagram when the ambient temperature T _o when the distortion ratio [rho and the distortion ratio [rho for A material 5 is plotted, and the coefficients a ₂ and b ₂ are obtained using the method of least squares. In the case of the example shown in FIG.

Was requested.

<Fitting of (Formula 3)>

Perform fitting.

First, as shown in FIG. 4A, for the same A material, uniaxial tension (ρ = −0.5), biaxial tension (ρ = 1.0), plane strain tension (ρ = 0). ), The data representing the relationship between the maximum principal strain ε ₁ (= breaking strain ε _f ) and the temperature T are plotted on the graph of the judgment strain ε _{f with} the horizontal axis representing the temperature T (° C.). Then, the maximum breaking strain temperature Tp (ρ) at which the maximum principal strain ε ₁ (= breaking strain ε _f ) connected by a smooth curve becomes the breaking strain maximum value is obtained.

Secondly, paying attention to a uniaxial tension curve, the maximum strain at which the maximum principal strain ε ₁ (= break strain ε _f ) becomes the maximum value at break strain T _p (ρ) is 150 ° C., and the maximum value is 0. 7. Read the inflection point at 75 ° C. and the initial strain as 0.4, obtain T _p (ρ) = 150, σ / k = 75, 1 + D = 0.7 / 0.4 = 1.75, _p (ρ) = 150, σ = 90, k = 1.2, D = 0.75,

Get.

<Fitting of (Formula 4)>

Perform fitting.
As shown in FIG. 4B, the obtained temperature Tp (ρ) and strain ratio ρ data are plotted, and straight lines a ₁ and b ₁ satisfying these are obtained.
About A material of FIG.4 (b),

Get. In the case of material B, which is a different material,

To get a different formula.

<Creation of warm FLD>
FIG. 3A shows a warm FLD-A created using the fitted (Formula 2), (Formula 8), (Formula 9) and (Formula 10).
FIG. 3B shows a warm FLD-B created for the material B in the same procedure.

The determination of cracks in the thin plate according to the second embodiment will be described with reference to Table 1.
In Table 1, 1-8 are examples of the present invention, and 9-14 are comparative examples.
Two types of materials A and B are used.

  FLD was prepared in consideration of the maximum value of fracture strain and the maximum temperature of fracture strain of steel materials containing austenite (γ), and FLD-A (for material A, indicated as “A” in Table 1), FLD-B (material Z and Z2 prepared without considering the breaking strain maximum value and the breaking strain maximum temperature of the steel material containing austenite (γ) are used.

  Here, FLD-A is the warm FLD in FIG. 3A, and FLD-B is the warm FLD in FIG. 3B.

  The stress-strain diagram shows SS-A (for material A, indicated as “A” in Table 1) and SS-, which were prepared in consideration of the maximum value of fracture strain and the maximum fracture strain temperature of steel materials containing austenite (γ). Z (Z1 and Z2) prepared without considering the breaking strain maximum value and breaking strain maximum temperature of B (for material B and indicated as “B” in Table 1) and austenite (γ) are used.

  The plate thickness, BHF, friction coefficient, forming speed, processing temperature 1, processing temperature 2, and processing temperature 3 are as described in Table 1.

  Formability evaluation 1, formability evaluation 2, and formability evaluation 3 were analyzed with the plate thickness, BHF, friction coefficient, and forming speed shown in Table 1 at processing temperature 1, processing temperature 2, and processing temperature 3, respectively. Is evaluated as “X” when it is determined that “cracking occurs” and “O” when it is determined that “no crack”.

  Comprehensive evaluation is “◎” if the crack judgment of the experiment and analysis is the same in 3 out of 3 conditions, “○” if the crack judgment of the experiment and analysis is in 2 out of the 3 conditions, When the crack determination is equal to or less than one of the three conditions, “x” is given.

  The A material cracked when molded at 25 ° C., but no crack occurred when molded at 150 ° C. and 175 ° C. Further, although the B material was cracked when molded at 25 ° C., it did not crack when molded at 75 ° C. and 65 ° C.

When the analysis results and the experimental results were collated under the above conditions, the results of Examples 1 to 8 were ◎ to ◯, and excellent effects were recognized, but in Comparative Examples 9 to 14, the results were x.
FIG. 7 shows the determination result of No. 1 (example of the present invention). When the method of the present invention is used, molding at room temperature (25 ° C.) and warm molding (150 ° C.) are performed. It is determined whether or not there is a crack.

When the method of the present invention is used, cracking occurs when molding is performed at room temperature (25 ° C.), but cracking does not occur in warm conditions (150 ° C.).

  The method can be used in a method for evaluating the warm formability of an elastic-plastic material by a warm FLD model based on the result of a press forming simulation of a thin plate using a finite element method.

DESCRIPTION OF SYMBOLS 1 Die 2 Blank holder 3 Punch 4 Draw bead 5 Work material 6 Direction of punch at the time of processing 11 Grazing part 12 of test machine Tensile specimen 13 Evaluation distance 14 Die direction 22 Die 22 Blank holder 23 Punch 24 Draw bead 25 Work material 26 Punch direction during processing 27 Constant temperature bath 28 Heater 29 Silicon oil 30 Blank holder holding spring 31 Cracking

Claims (5)

A method for producing a warm FLD used for cracking evaluation of a steel material containing austenite (γ),
For each strain ratio of the steel material containing austenite (γ), the maximum value of the breaking strain (hereinafter referred to as “breaking strain maximum value”) and the temperature at which the breaking strain maximum value is obtained (hereinafter referred to as “breaking strain maximum temperature”). A method for evaluating cracking of a thin plate, comprising the step of creating a warm FLD having information on A method of making the warm FLD,
About a steel material containing one predetermined austenite (γ),
A step of collecting a plurality of data of temperature T and breaking strain ε _f for each of a plurality of strain ratios ρ;
Creating a curve satisfying a plurality of data of the temperature T and the breaking strain ε _f for each strain ratio ρ;
The variable is one temperature, convex upward, has an inflection point and a single maximum value, the temperature that takes the maximum value changes from the strain ratio ρ, and passes through the temperature value that takes the maximum value. A step of fitting a symmetric function (hereinafter referred to as “one-variable symmetric function”) with respect to a straight line perpendicular to the temperature axis to any one of the plurality of curves (hereinafter referred to as “FLD function”). And)
Wherein the plurality of strain at break up to a temperature T _[rho and the curve distortion ratio curve [rho, creating a function T _{p ([rho)} of the Dan'ibitsu maximum temperature T _p fracture of the plurality of curves the Ibitsuhi [rho variable When,
The method for evaluating cracks in a thin plate according to claim 1, wherein: A method of making the warm FLD,
About the steel material containing one predetermined austenite (γ),
For a plurality of strain ratios ρ, collecting fracture strain ε _f0 data at room temperature T ₀ ;
Creating a function (hereinafter referred to as “room temperature rupture strain function”) for calculating a rupture strain ε _f0 at room temperature T ₀ when a strain ratio ρ is given from the data;
The method for evaluating cracking of a thin plate according to claim 2, wherein: A method of creating the normal temperature breaking strain function,
A function f () having the work hardening index n as a strain ratio ρ variable from the data of the work hardening index n and the break strain ε _{f0 at} the normal temperature T ₀ from the plurality of strain ratios ρ and the breaking strain ε _f0 data at the room temperature T _0. creating ρ);
Using f (ρ) to create a normal temperature breaking strain function;
The method for evaluating cracking of a thin plate according to claim 3, wherein:   A method for evaluating cracking of a thin plate, wherein the warm formability is determined based on a result obtained by a finite element method using the warm FLD obtained by the method according to claim 1.